This invention relates to methods and apparatus for depositing silicon dioxide onto a substrate, to substrates having silicon dioxide deposited thereon, and also to structures and devices incorporating same.
Sputtered silicon dioxide has been widely used as a dielectric insulator of electronic switching and sensing devices such as metal-insulator-metal and metal-insulator-semiconductor switching devices. Silicon dioxide is also very attractive in optical and acoustic applications such as SAW, BAW and TCSAW filters because of its low refractive index, high transparency, and positive TCF (temperature coefficient of frequency).
It is known to deposit silicon dioxide thin films by thermal oxidation, plasma enhanced chemical vapour deposition (PECVD), reactive RF sputtering, and pulsed DC sputtering. The technique used to deposit the thin film is generally selected so that the properties of the thin film are best suited to the intended end application. Sputter deposition has the distinct advantage of being a low temperature fabrication process. In addition, pulsed DC magnetron sputtering provides higher deposition rates, more repeatable performance, and improved film quality compared to complicated and expensive RF sputtering techniques. However, there are certain drawbacks associated with pulsed DC sputtering. Conventional pulsed DC reactive sputtering of silicon dioxide and PECVD of silicon dioxide result in films having a density of less than or equal to 2.30 gcm−3. However, it would be very desirable to produce silicon dioxide films having a density of greater than 2.30 gcm−3 because this gives rise to improved acoustic and electrical performance such as in TCSAW applications.
It is known that the density of reactively sputtered silicon dioxide films can be increased by applying a DC bias to the wafer that the film is reactively sputtered onto. However, the maximum silicon dioxide film density achievable with conventional Ar sputtering is limited to about 2.35 gcm−3. In addition to a desire to provide high density silicon dioxide films, there is also an ongoing commercial impetus to provide improved throughput during fabrication.